Tellurapyrylium dyes are of interest for a variety of electrophotographic, optical recording, and other applications. These dyes can have a variety of substituents, and can be prepared in a number of ways.
A dye having a methine or a trimethine bridge, with a tellurapyrylium nucleus at one end, and a pyrylium, thiapyrylium or selenapyrylium nucleus at the other end of the bridge, will be referred to herein as an "unsymmetrical tellurapyrylium dye," or as "UTPD". Such dyes, with a methine bridge, can be made by reacting a 2- or 4-methyltellurapyrylium nucleus with a chalcogenapyranone. Likewise, unsymmetrical tellurapyrylium dyes (UTPD) with a trimethine bridge can be made by a similar reaction, wherein the chalcogenapyranone is replaced with a chalcogenapyranylacetaldehyde. For these reactions, a solvent of choice is a carboxylic acid anhydride, such as acetic anhydride.
The above-mentioned processes comprise the reaction of an active methyl compound with a carbonyl compound, and can be considered a type of aldol process. The reactions are not straightforward.
The processes generate an olefinic double bond and a molecule of water. The water which is formed leads to process complications. More specifically, water catalyzes a reverse aldol process which leads to the formation of symmetrical dyes, along with the desired unsymmetrical dye. The symmetrical co-products are referred to herein as "analogs" of the unsymmetrical dye.
By way of illustration, the condensation of 2,6-di-tert-butyl-4-methyltellurapyrylium hexafluorophosphate with 2,6-di-tert-butyl-4-selenapyran-4-one, gives a 95:5 (molar basis) mixture of the tellurapyrylium-4-(selenapyrilidene) methyl dye and the symmetrical selenapyrylium-4-(selenapyrilidene) methyl dye. This is depicted by Equation (1) wherein Me signifies --CH.sub.3, and t-Bu signifies tert-butyl. ##STR1##
Similarly, condensation of the same 4-methyl compound with a (chalcogenapyranylidene)acetaldlehyde gives the unsymmetrical trimethine dyes shown in Equation (2): ##STR2##
Reverse aldol reactions also occur during the synthesis of these trimethine bridged compounds. Preparation of the unsymmetrical tellurium/selenium trimethine dye illustrated above using acetic anhydride as a solvent gave a mixture consisting of 92% of the desired dye, 7% of the symmetrical selenium/selenium trimethine dye, and 1% of the symmetrical tellurium/tellurium trimethine dye. Preparation of the unsymmetrical tellurium/sulfur trimethine dye gave a mixture containing 88% of the desired dye and 12% of the symmetrical sulfur/sulfur trimethine dye. Preparation of the unsymmetrical tellurium/oxygen trimethine dye gave a mixture containing 85% of the desired dye and 15% of the symmetrical oxygen/oxygen trimethine dye.
If desired, these dye mixtures can be purified by careful recrystallization or by preparative liquid chromatography. In both cases, purification is tedious and not amenable to large scale purification.
Relative to the lighter halogens, selenium, sulfur, and oxygen, tellurium undergoes a very rapid oxidative addition with halogens. Even tellurapyrylium dyes will undergo oxidative addition of halogens to give tellurium(IV) containing species. These tellurium(IV) containing species are very insoluble relative to the starting tellurapyrylium species.
Thus, this invention provides a purification technique which is simple to carry out a more efficacious or easier to conduct than the previous methods mentioned above. Consequently, this invention is considered to be a significant advance in the art.